1. Field of the Invention
The present invention relates to an electron generating apparatus in which a plurality of surface conduction electron-emitting devices are arranged, an image forming apparatus using the electron generating apparatus, and a method of manufacturing and adjusting the same.
2. Related Background Art
Conventionally, two types of devices, namely thermionic and cold cathode devices, are known as electron-emitting devices. Examples of cold cathode devices are surface conduction electron-emitting devices, field emission type devices (to be referred to as FE type devices hereinafter), and metal/insulator/metal type emission devices (to be referred to as MIM type devices hereinafter).
Known examples of the FE type devices are described in W. P. Dyke and W. W. Dolan, "Field Emission", Advance in Electron Physics, 8,89 (1956) and C. A. Spindt, "Physical properties of thin-film field emission cathodes with molybdenum cones", J. Appl. Phys., 47,5248 (1976).
A known example of the MIM type devices is described in C. A. Mead, "Operation of Tunnel-emission Devices", J. Appl. Phys., 32,646 (1961).
A known example of the surface conduction electronemitting devices is described in, e.g., M. I. Elinson, Radio. Eng. Electron Phys., 10, 1290 (1965) and other examples to be described later.
The surface conduction electron-emitting device utilizes the phenomenon that electron emission is caused in a small-area thin film, formed on a substrate, by passing a current parallel to the film surface. The surface conduction electron-emitting device includes devices using an Au thin film (G. Dittmer, "Thin Solid Films", 9,317 (1972)), an In.sub.2 O.sub.3 /SnO.sub.2 thin film (M. Hartwell and C. G. Fonstad, "IEEE Trans. ED Conf.", 519 (1975)), and a carbon thin film (Hisashi Araki, et al., "Vacuum", Vol. 26, No. 1, p. 22 (1983)), and the like, in addition to an SnO.sub.2 thin film according to Elinson mentioned above.
FIG. 25 is a plan view of the surface-conduction emitting device according to M. Hartwell et al. as a typical example of the structures of these surface conduction electron-emitting devices. Referring to FIG. 25, reference numeral 3001 denotes a substrate; and 3004, a conductive thin film made of a metal oxide formed by sputtering. This conductive thin film 3004 has an H-shaped pattern, as shown in FIG. 25. An electron-emitting portion 3005 is formed by performing an electrification process (referred to as an energization forming process to be described later) with respect to the conductive thin film 3004. Referring to FIG. 25, a spacing L is set to 0.5 to 1 [mm], and a width W is set to 0.1 [mm]. The electron-emitting portion 3005 is shown in a rectangular shape at the center of the conductive thin film 3004 for the sake of illustrative convenience, however, this does not exactly show the actual position and shape of the electron-emitting portion.
In the above surface conduction electron-emitting device by M. Hartwell et al., typically the electron-emitting portion 3005 is formed by performing the electrification process called the energization forming process for the conductive thin film 3004 before electron emission. According to the energization forming process, electrification is performed by applying a constant DC voltage which increases at a very slow rate of, e.g., 1 V/min, to both ends of the conductive thin film 3004, so as to partially destroy or deform the conductive thin film 3004 or change the properties of the conductive thin film 3004, thereby forming the electron-emitting portion 3005 with an electrically high resistance. Note that the destroyed or deformed part of the conductive thin film 3004 or part where the properties are changed has a fissure. Upon application of an appropriate voltage to the conductive thin film 3004 after the energization forming process, electron emission is performed near the fissure.
The above surface conduction electron-emitting devices are advantageous because, of cold cathode devices, they have a simple structure and can be easily manufactured. For this reason, many devices can be formed on a wide area. As disclosed in Japanese Patent Laid-Open No. 64-31332 filed by the present applicant, a method of arranging and driving a lot of devices has been studied.
Regarding applications of surface conduction electron-emitting devices to, e.g., image forming apparatuses such as an image display apparatus and an image recording apparatus, charged beam sources and the like have been studied.
As an application to image display apparatuses, in particular, as disclosed in U.S. Pat. No. 5,066,883 and Japanese Patent Laid-Open Nos. 2-257551 and 4-28137 filed by the present applicant, an image display apparatus using the combination of a surface conduction electron-emitting device and a phosphor which emits light upon irradiation of an electron beam has been studied. This type of image display apparatus is expected to have more excellent characteristics than other conventional image display apparatuses. For example, in comparison with recent popular liquid crystal display apparatuses, the above display apparatus is superior in that is does not require a backlight since it is of a light emissive type and that it has a wide view angle.
The present inventors have examined cold cathode devices according to various materials, manufacturing methods, and structures, in addition to the above conventional devices. The present inventors have also studied a multi-electron-beam source in which a lot of cold cathode devices are arranged, and an image display apparatus to which this multi-electron-beam source is applied.
The present inventors have also examined a multi-electron-beam source according to an electric wiring method shown in FIG. 26. More specifically, this multi-electron-beam source is constituted by two-dimensionally arranging a large number of cold cathode devices and wiring these devices in a matrix, as shown in FIG. 26.
Referring to FIG. 26, reference numeral 4001 denotes a cold cathode device; 4002, a row wiring layer; and 4003, a column wiring layer. The row wiring layers 4002 and the column wiring layers 4003 actually have limited electrical resistances which are represented as wiring resistances 4004 and 4005 in FIG. 26. The wiring shown in FIG. 26 is referred to as simple matrix wiring. For the illustrative convenience, the multi-electron-beam source constituted by a 6.times.6 matrix is shown in FIG. 26. However, the scale of the matrix is not limited to this arrangement, as a matter of course. In a multi-electron-beam source for an image forming apparatus, a number of devices sufficient to perform desired image display are arranged and wired.
In the multi-electron-beam source in which the surface conduction electron-emitting devices are wired in a simple matrix, appropriate electrical signals are supplied to the row wiring layers 4002 and the column wiring layers 4003 to output desired electron beams. When the surface conduction electron-emitting devices of an arbitrary row of the matrix are to be driven, a selection voltage Vs is applied to the row wiring layer 4002 of the selected row. Simultaneously, a non-selection voltage Vns is applied to the row wiring layers 4002 of unselected rows. In synchronism with this operation, a driving voltage Ve for outputting electron beams is applied to all the column wiring layers 4003. According to this method, a voltage (Ve-Vs) is applied to the surface conduction electron-emitting devices of the selected row, and a voltage (Ve-Vns) is applied to the surface conduction electron-emitting devices of the unselected rows, assuming that a voltage drop caused by the wiring resistances 4004 and 4005 is negligible. When the voltages Ve, Vs, and Vns are set to appropriate levels, electron beams with a desired intensity are output from only the surface conduction electron-emitting devices of the selected row. When different driving voltages Ve are applied to the respective column wiring layers 4003, electron beams with different intensities are output from the respective devices of the selected row. Since the response rate of the surface conduction electron-emitting device is fast, the period of time over which electron beams are output can also be changed in accordance with the period of time for applying the driving voltage Ve.
The multi-electron-beam source having surface conduction electron-emitting devices arranged in a simple matrix can be used in a variety of applications. For example, the multi-electron-beam source can be suitably used as an electron source for an image forming apparatus by appropriately supplying an electrical signal according to image information.
As a result of extensive studies for improving the characteristics of the surface conduction electron-emitting device, the present inventors found that an activation process in the manufacturing process was effective.
As described above, when the electron-emitting portion of the surface conduction electron-emitting device is to be formed, a process (energization forming process) of flowing a current to the conductive thin film to locally destroy, deform, or deteriorate the thin film and form a fissure is performed. Thereafter, when the activation process is performed, the electron-emitting characteristics can be largely improved. More specifically, the activation process is a process of performing electrification of the electron-emitting portion formed by the energization forming process, under appropriate conditions, to deposit carbon or a carbon compound around the electron-emitting portion. For example, a predetermined voltage pulse is periodically applied in a vacuum atmosphere in which an organic substance at an appropriate partial pressure exists, and the total pressure is 10.sup.-4 to 10.sup.-5 [Torr]. With this process, any of monocrystalline graphite, polycrystalline graphite, amorphous carbon, and a mixture thereof is deposited near the electron-emitting portion to a thickness of about 500 [.ANG.] or less. These conditions are only examples and must be appropriately changed in accordance with the material and shape of the surface conduction electron-emitting device.
With this process, comparing the electron-emitting portion with that before the activation process, the emission current at the same applied voltage can be increased typically about 100 times or more. Therefore, in manufacturing a multi-electron-beam source using a lot of surface conduction electron-emitting devices as well, the activation process is preferably performed for each device.
After the activation process is completed, for the purpose of stabilizing the electron-emitting characteristics of the surface conduction electron-emitting device, the partial pressure of an organic gas in the vacuum atmosphere around the surface conduction electron-emitting device is reduced, thereby preventing further deposition of carbon or a carbon compound at the electron-emitting portion or its peripheral portion even when a voltage is applied to the surface conduction electron-emitting device, and this state must be maintained. Preferably, the partial pressure of the organic gas in the atmosphere is reduced to 10.sup.-8 [Torr] or less, and this state is maintained. If possible, the partial pressure is preferably maintained at 10.sup.-10 [Torr] or less. Note that the partial pressure of the organic gas is obtained by integrating the partial pressures of organic molecules having carbon and hydrogen as major ingredients and having a mass number of 13 to 200, which is quantitatively measured using a mass spectrograph.
A typical method of reducing the partial pressure of the organic gas around the surface conduction electron-emitting device is as follows. The vacuum vessel incorporating the substrate on which the surface conduction electron-emitting device is formed is heated. While desorbing the organic gas molecules from the surface of each member in the vessel, vacuum evacuation is performed using a vacuum pump such as a sorption pump or an ion pump using no oil. After the partial pressure of the organic gas is reduced in this manner, this state can be maintained by continuously performing evacuation using the vacuum pump with no oil. However, this method using the vacuum pump for continuous evacuation has disadvantages in volume, power consumption, weight, and cost depending on the application purpose. When the surface conduction electron-emitting device is to be applied to an image display apparatus, the organic gas molecules are sufficiently desorbed to reduce the partial pressure of the organic gas, and thereafter, a getter film is formed in the vacuum vessel, and at the same time, the exhaust pipe is sealed, thereby maintaining the state.
With this process, neither carbon nor carbon compound are newly deposited by electrification or a change in the surface conduction electron-emitting device with the elapse of time after the activation process, so that the electron-emitting characteristics can be stabilized.
As described above, measures for improving and stabilizing the electron-emitting characteristics of the surface conduction electron-emitting device are taken, though the multi-electron-beam source using the surface conduction electron-emitting device has the following problem.
In some cases, the peak value of a voltage applied to drive the multi-electron-beam source increases due to the temperature characteristic (e.g., a temperature drift) of the driving circuit, or instantaneously increases due to a disturbance (e.g., noise or static electricity of the circuit), as shown in FIG. 3. When this increase in voltage value increases the peak value of the driving voltage beyond a predetermined value (the largest one of voltage values applied to the multi-electron-beam source previously), the device characteristics of the surface conduction electron-emitting device change immediately after the voltage is applied to the multi-electron-beam source. For this reason, even when the same voltage as that before the change in characteristics of the surface conduction electron-emitting device of the multi-electron-beam source is applied, the electron emission amount changes (decreases). When the multi-electron-beam source is applied to an image display apparatus, the luminance of a row where an image is displayed during the driving operation decreases, resulting in, e.g., a luminance variation in the row direction of the display image.